Cell shrinkage is an early prerequisite for apoptosis. The apoptotic volume decrease is due primarily to loss of cytoplasmic ions. Increased outward K⁺ currents have indeed been implicated in the early stage of apoptosis in many cell types. We found that cytoplasmic dialysis of cytochrome c(cyt-c), a mitochondria-dependent apoptotic inducer, increases K⁺ currents before inducing nuclear condensation and breakage in pulmonary vascular smooth muscle cells. The cyt-c-mediated increase in K⁺ currents took place rapidly and was not affected by treatment with a specific inhibitor of caspase-9. Cytoplasmic dialysis of recombinant (active) caspase-9 negligibly affected the K⁺ currents. Furthermore, treatment of the cells with staurosporine (ST), an apoptosis inducer that mediates translocation of cyt-c from mitochondria to the cytosol, also increased K⁺ currents, caused cell shrinkage, and induced apoptosis (determined by apoptotic nuclear morphology and TdT-UTP nick end labeling assay). The staurosporine-induced increase in K⁺ currents concurred to the volume decrease but preceded the activation of apoptosis (nuclear condensation and breakage). These results suggest that the cyt-c-induced activation of K⁺ channels and the resultant K⁺ loss play an important role in initiating the apoptotic volume decrease when cells undergo apoptosis.